G35

2-3 layers, moss layer 5-10 cm thick, herbaceous /dwarf shrub layer 20-50 cm tall, sometimes with low shrub layer to 80 cm.

a Sum of monthly mean temperatures above 0 °C. Source: Based on Elvebakk (1999).

extinctions, migrations and invasions. However, even at the height of the Pleistocene glaciations the Arctic was never entirely covered in ice. The extent of the Pleistocene glaciations in subarctic regions in western Europe (Fig. 6.11) can lead to an erroneous assumption that all the land north of the Arctic Circle must have also been subjected to a prolonged and deep ice cover, with the inevitable consequence that terrestrial plant life would have been extinguished - the tabula rasa effect. It would follow from this assumption that the entire present-day arctic flora must be the result of recent immigration from more southerly latitudes, as has more probably been the case for many insect species (Buckland & Dugmore, 1991).

The evidence that has been used in earlier discussions of whether hardy refugial populations of arctic plants survived in ice-free areas and nunataks during the Last Glacial Maximum has rested mainly on indirect arguments based on phytogeographical evidence. On this question of arctic refugia it is necessary to distinguish carefully between inland nunataks on high mountain chains as in Scandinavia, where serious doubts have been raised as to their role as refugia (Nordal, 1987), and coastal low altitude nunataks at the edge of ice sheets. In coastal areas, mountains to seaward of stable ice sheets need be of only moderate height to rise above terminal ice cliffs. There are many such areas where these terminal ice cliffs, or semi-nunataks (Figs. 6.12-6.13) as they are sometimes called, could have provided Pleistocene refugia in North America as well as in south Iceland and the islands of the Arctic Ocean.

Earlier estimates of the past extent of polar ice were mostly based on oceanic sedimentation studies (Mangerud et al., 1998) which suggested that from Spitsbergen to northern Norway there was extensive sea ice cover adjacent to the coast. However, research using rock-exposure dating techniques based on cosmic-ray-produced isotopes has now provided a reassessment of the extent of Pleistocene ice cover which has downgraded earlier estimations of both the depth of the ice and its seaward extension. Consequently, there are various sites where the lower ice levels indicate the existence of nunataks in western Scotland, Norway (Ballantyne et al., 1998) and also in Spitsbergen (Landvik et al., 2003).

In northern Norway the island of Andoya has long been the subject of geological and botanical investigations as its situation close to the continental shelf has prompted the suggestion that it supported a possible Late Weichselian unglaciated enclave. Early deglaciation of the island has now been clearly recognized, as

Fig. 6.10 Maps illustrating a sharp pH boundary along the northern front of the arctic foothills in Alaska which separates the non-acidic (pH>6.5) ecosystems to the north from the predominantly acidic (pH 5.5) ecosystems to the south. (a) Landsat false-colour infrared mosaic of the Kuparuk River Basin, northern Alaska. The pH boundary (dashed line) separates the redder tones to the south of the acidic soils from the greyer tones to the north of the non-acidic soils. (b) Generalized distribution of acidic and non-acidic vegetation types in northern Alaska. (c) The location of the pH boundary west of the Colville River (white dashed line) is less distinct and has therefore attracted less investigation. (Reproduced with permission from Walker et al, 1998.)

Fig. 6.10 Maps illustrating a sharp pH boundary along the northern front of the arctic foothills in Alaska which separates the non-acidic (pH>6.5) ecosystems to the north from the predominantly acidic (pH 5.5) ecosystems to the south. (a) Landsat false-colour infrared mosaic of the Kuparuk River Basin, northern Alaska. The pH boundary (dashed line) separates the redder tones to the south of the acidic soils from the greyer tones to the north of the non-acidic soils. (b) Generalized distribution of acidic and non-acidic vegetation types in northern Alaska. (c) The location of the pH boundary west of the Colville River (white dashed line) is less distinct and has therefore attracted less investigation. (Reproduced with permission from Walker et al, 1998.)

has also the existence on Andoya of ice-free areas with possible nunataks on the nearby islands of Senja and Grytoya. Geomorphological mapping suggests that

Fig. 6.11 Map for the northern hemisphere at the Last Glacial Maximum (Weichselian, Valdaian, Würmian, Devensian, Wisconsinan, MIS 2) compiled by Jürgen Ehlers from data and maps assembled and published in Quaternary Glaciations (Ehlers & Gibbard, 2004).

the upper surface of the Late Weichselian ice sheet at the Last Glacial Maximum rose eastwards from sea level on north-west Andoya to c. 1200 m in the longitude of Narvik (17° E), and that numerous mountain summits remained above the ice as nunataks (Vorren et al, 1988).

Spitsbergen provides another well-studied example of ice-free terrain at high latitudes (Fig. 6.12). Using the isotope dating technique of the exposure of rock surfaces to cosmic radiation, it has been shown that the upper levels of late Weichselian ice sheet on the north-west islands of Amsterdamoya and Danskoya (79° 45' N) have had ice-free nunataks since the Last Glacial Maximum. More extensive ice-free areas have also been postulated along the west coast of Spitsbergen during the last glaciation by studies of radiocarbon dates in mollusc shells and whalebone (Landvik et al., 2003). A typical west coast Spitsbergen nunatak is shown in Fig. 6.13.

The present-day arctic flora is thought to have originated during the late Tertiary period approximately 3 million years ago, partially from some species of the previous arctic forest biome, but also from mountain species in subarctic regions which then migrated north as

Biome During Last Glacial Maximum

♦ Bedrock • Glacial erratic A Autochthonous block field

Fig. 6.12 Topographic profiles of Amsterdamoya and Danskoya (see location in inset) with dates for the exposure ages of sampled rocks in ka. The dotted lines are estimates of the minimum and maximum elevations of the Weichselian ice sheet. (Reproduced with permission from Landvik et al., 2003.)

♦ Bedrock • Glacial erratic A Autochthonous block field

Fig. 6.12 Topographic profiles of Amsterdamoya and Danskoya (see location in inset) with dates for the exposure ages of sampled rocks in ka. The dotted lines are estimates of the minimum and maximum elevations of the Weichselian ice sheet. (Reproduced with permission from Landvik et al., 2003.)

Fig. 6.13 A probable former semi-nunatak (red arrow) on the Casimir Perierkammen in Krossfjorden (north-west Spitsbergen). This is one of a number of regions in north-west Svalbard considered by a number of authors to have been free of ice at various times during the Weichselian glaciation. Such areas could have been glacial refugia sites (see Ingolfson & Forman, 1997). Note also the remains in the foreground of a former Russian (Pomor) hunting camp now colonized by mosses.

Fig. 6.13 A probable former semi-nunatak (red arrow) on the Casimir Perierkammen in Krossfjorden (north-west Spitsbergen). This is one of a number of regions in north-west Svalbard considered by a number of authors to have been free of ice at various times during the Weichselian glaciation. Such areas could have been glacial refugia sites (see Ingolfson & Forman, 1997). Note also the remains in the foreground of a former Russian (Pomor) hunting camp now colonized by mosses.

the climate became cooler, forests retreated from the Arctic and the present tundra conditions developed.

The dates now available for the retreat of the ice sheets at high latitudes show that timing and extent of the ice cover at the Last Glacial Maximum varied considerably with location. Estimates of the extent of ice cover during the Last Glacial Maximum range from those who consider that there was only limited glacial extension in arctic Canada and northern Eurasia (Pavlidis et al., 1997), to others who maintain that the cover was more widespread (Grosswald, 1998). The minimalist view also asserts that glacial maxima were not synchronous throughout the northern hemisphere; for example, the Novaya Zemlya ice sheet reached its maximum extent c. 39 000 BP, approximately 20 000 years earlier than the maximum extent of the contiguous Scandinavian ice sheet. Even during the coldest periods of the Pleistocene, glaciation did not extend to all areas of arctic Russia.

The rain shadow area of north-eastern Siberia has been relatively ice free throughout the Last Glacial Maximum, and traces have been found there of where Palaeolithic man was hunting mammoth 40 000 years ago. Plant macrofossils found on the Bykovsky Peninsula (west Beringia) indicated the existence of productive meadow and steppe communities during the late Pleistocene and could have served as a food resource for large populations of herbivores (Kienast et al., 2005). Other areas further west where the climate was more oceanic became free of ice only much later.

However, even here there were areas which are now known to have been free of ice at the Last Glacial Maximum. Consequently, when discussing the arctic flora it is essential to remember that the evolutionary history of the vegetation varies greatly from one region to another.

The Taymyr Peninsula, like much of north-eastern Siberia came into a precipitation shadow and was consequently ice free during times when there was large-scale ice coverage in Scandinavia and north-western Europe (Moller et al., 1999). A case can also be made for ice-free refugia in the south-west region of the South Island of Novaya Zemlya on the basis of its proximity to the edge of the Last Glacial Maximum ice boundaries (Tveranger et al., 1999) and the demonstration that pollen and radiocarbon data on the western coast of Novaya Zemlya are sufficient to disprove earlier theories of extensive glaciation during the Upper Pleistocene (Serebryanny & Malyasova, 1998). In the lands to the east, both western and central Siberia and the Chukotsky Peninsula were all marked by limited glaciation during the Last Glacial Maximum (Velichko et al., 1997). The dry, cold climate of Asia during the Pleistocene was associated with a specific tundra-steppe biome which was floristically, historically, and ecologically distinct from the polar deserts of the more high latitude sites.

Figure 6.14 shows some of the sites where there is now a corpus of mainly geological evidence for ice-free areas in the Arctic at the time of the Last Glacial Maximum which makes it probable that there were

Biome During Last Glacial Maximum

Fig. 6.14 Location of some sites within the Arctic that were ice-free and may have served as refugia for flowering plants during at least part of the Last Glacial Maximum. All examples are based on geological or palaeological evidence with the exception of south Greenland (Arsuk and Kap Farvel) and south Iceland where the evidence is phytogeographical. (For sources of information see Crawford, 2004.)

Fig. 6.14 Location of some sites within the Arctic that were ice-free and may have served as refugia for flowering plants during at least part of the Last Glacial Maximum. All examples are based on geological or palaeological evidence with the exception of south Greenland (Arsuk and Kap Farvel) and south Iceland where the evidence is phytogeographical. (For sources of information see Crawford, 2004.)

numerous outlying populations of flowering plants surviving, at this time north of the major ice sheets.

6.4.2 Molecular evidence for the existence of glacial refugia at high latitudes

To investigate the hypothesis that plants survived north of the major ice sheets a major investigation was undertaken of the widespread polymorphic and highly successful arctic colonizer the opposite-leaved purple saxifrage (Saxífraga oppositifolia). The species was sampled for its chloroplast DNA variation at a number of sites throughout its circumpolar distribution (Fig. 6.15) with the aim of establishing how this species colonized the Arctic during the late Tertiary or early Pleistocene and whether or not refugia for this and other species occurred in the Arctic as well as at more southern latitudes during the Pleistocene ice ages. Fourteen different haplotypes (A-N) of chloroplast DNA (cpDNA) were identified. Phylogenetic analysis showed that S. oppositifolia is composed of two major DNA lineages: a 'Eurasian lineage' distributed westward from the Tay-myr Peninsula in north-central Siberia, through Europe and Greenland to Newfoundland and Baffin Island and a 'North American' lineage, distributed eastward from the Taymyr Peninsula, through north-east Siberia and North America to North Greenland. Haplotypes basal to each lineage co-occur only in north-central

Fig. 6.15 Summary of results from a survey of chloroplast DNA variation in purple saxifrage (Saxifraga oppositifolia). This study was conducted to reconstruct the evolutionary history of this circumpolar arctic plant and to establish how this species colonized the Arctic during the late Tertiary/early Pleistocene, and whether refugia for this and other arctic plants occurred in the Arctic as well as at more southern latitudes during Pleistocene ice ages — for detail see text. (Adapted with permission from Abbott & Brochmann, 2003.)

Fig. 6.15 Summary of results from a survey of chloroplast DNA variation in purple saxifrage (Saxifraga oppositifolia). This study was conducted to reconstruct the evolutionary history of this circumpolar arctic plant and to establish how this species colonized the Arctic during the late Tertiary/early Pleistocene, and whether refugia for this and other arctic plants occurred in the Arctic as well as at more southern latitudes during Pleistocene ice ages — for detail see text. (Adapted with permission from Abbott & Brochmann, 2003.)

Fig. 6.16 Circumpolar distribution of Saxífraga oppositifolia and the location of the Sino-Himalyan region where other opposite-leaved species of the sect. Porphyrion subsection oppositifoliae also occur. (Map adapted with permission from Webb & Gornall, 1989.)

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Fig. 6.16 Circumpolar distribution of Saxífraga oppositifolia and the location of the Sino-Himalyan region where other opposite-leaved species of the sect. Porphyrion subsection oppositifoliae also occur. (Map adapted with permission from Webb & Gornall, 1989.)

Siberia (Taymyr). A possible explanation of this phylo-geographic phenomenon is that S. oppositifolia first occurred in the Arctic in western Beringia in the Sino-Himalayan region (Fig. 6.16) during the late Tertiary before migrating east and west to obtain complete circumpolar distribution (Abbott et al., 2000) and could have migrated northwards along the Altai Mountains to reach the shores of the Arctic Ocean.

Saxifraga oppositifolia has long been regarded as one of the ancient species of the arctic flora (Tolmatchev, 1966), largely maintaining the diploid state over a wide geographical range together with much morphological variation. Tetraploid cytotypes are found, but the relative distribution of the two cytotypes is not known. In Spitsbergen it is claimed that only the diploid type is present (Brysting et al., 1996).

Other species of opposite-leaved saxifrages can be found in the Altai Mountains. It would therefore appear from the pattern of cpDNA distribution that S. oppositifolia is probably derived from ancestral stock native to high mountains in central Asia. From these ancestral populations the species migrated north during the late Tertiary to northern Siberia along mountain ranges that connect these two regions, and then spread around the shores of the Arctic Ocean.

The high cpDNA diversity present in Alaskan populations of S. oppositifolia (Fig. 6.15) supports fossil evidence that a major refugium for arctic plants was present in eastern Beringia during the last full glacial period. The possibility that a northern refugium also existed in unglaciated parts of the Canadian Arctic and north Greenland is not excluded but requires fossil evidence for confirmation. Similarly, it appears equally feasible that an arctic refugium occurred in parts of the Taymyr Peninsula which were never glaciated. Low cpDNA diversity is evident throughout the distribution of the mainly Eurasian lineage, which occupies mostly areas that were heavily glaciated during the last ice age. It is likely that migrants from the south, west and east of the main ice sheets colonized much of Europe, north-west Russia, Iceland, Greenland and north-east America during the post-glacial period (Abbott et al., 2000).

6.4.3 Evidence for an ancient (autochthonous) arctic flora

The improved knowledge of past migration routes and locations of important Pleistocene refugia for arctic plants adds substance to earlier phytogeographical speculations concerning the history of the arctic flora. Taxonomists who have paid particular attention to the arctic flora and its relationship to its Eurasian elements have long argued that not all species of arctic plants are of the same age. In areas where there has been little glaciation, as in eastern Siberia, Beringia and Alaska, there are species that have had a presence in the Arctic throughout the Pleistocene. Species with widespread distribution and chromosome counts that are simple diploids with no evidence of allo-polyploidization are likely to be ancient and indigenous members of an arcto-alpine flora that probably evolved on mountain chains outside the Arctic. The circumpolar species Saxifraga oppositifolia (2n — 26) is included in Tolmatchev's list (Table 6.2) of widespread arcto-alpine species; it has varying ecological forms but is probably not of arctic origin (Tolmatchev, 1966).

The cpDNA migrational history for S. oppositifolia described above provides substantial molecular evidence for the apparent long-term presence in the High Arctic of this particular species. Further research is needed to determine if the same molecular evidence for long-term residence in the Arctic will be found in the other species listed by Tolmatchev as autochthonous. With no total purging (the Pleistocene tabula rasa effect), it is to be expected that the Arctic flora as a whole will have had diverse origins, with some species surviving north of the ice sheets at the Last Glacial Maximum (e.g. Saxifraga oppositifolia) and then extending southwards as the ice retreated. The Arctic flora although lacking the species numbers found in warmer climates is nevertheless highly heterogeneous, not only in its genetic structure but also in its biogeographical history.

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